Recent advances in powder metallurgy processing techniques have permitted specialized applications, such as in the aerospace and nuclear energy industries where rigorous mechanical properties and high quality are required. These processing techniques include selecting and producing the proper alloy powder, consolidation, presintering, sintering and post-consolidation forming. See Metals Handbook, 9th Edition, Vol. 7, "Powder Metallurgy", American Society for Metals, (1984), and Metals Handbook, 8th Edition, Vol. 4, "Forming", American Society of Metals, (1969), which volumes are hereby incorporated by reference.
For part designs which require higher mechanical strength and greater densities pre-alloyed powders, such as ANCORSTEEL 1000B and 4600V (Hoeganaes Corporation), are often the material of choice. These powders can be produced by water atomization of molten metal and have a homogeneous composition.
In a conventional powder metallurgy processing, iron based powders are mixed with a lubricant and graphite, and alloying additions, prior to compaction. Typical compaction pressures range from about 25 to about 70 tsi (tons per square inch) with a resulting green density of about 6.3 to about 7.0 g/cm.sup.3.
Presintering, as it is known in the metallurgical arts, can be used to "delube" or burn off the admixed lubricant from the "green" compact and to impart sufficient strength to the green compact for handling. Usually, a delubing presinter is conducted at temperatures of about 430.degree.-650.degree. C. for about 30 minutes. Metals Handbook, 9th Edition, pp. 683. Presintering has also been employed at temperatures above about 2000.degree. F. (1090.degree. C.) for increasing the density of pure iron compacts by closing up large pores prior to sintering. Metals Handbook, 8th Edition, pp. 455-59.
Following presintering, repressing can be provided to the presintered preform where compaction is carried out similarly to the initial compaction step. The die and/or preform are usually lubricated.
The preform can then be sintered employing a continuous or batch-type sintering furnaces in dissociated ammonia for up to about one hour at 1090.degree.-1320.degree. C. (2000.degree.-2400.degree. F.).
While in the main, these conventional processing techniques for double pressed--double sintered iron powder have provided some increases in density and attendant mechanical properties, there remains a need for further improvement for specialized applications.